Oh god. Those dudes are crazy. Don't look him in the eye.
Wait until he gives his "And that's not all offer" at the least before showing any interest. If you're patient he'll inevitably double your order and throw in some kitchen doohicky worth thirty credits absolutely free! But it isn't until he says "And if you buy right now I'll cut one of the payments!" do you buy. I mean really, it's only two payments of 19.95. You can afford that, can't you?